In general, a coated paper for printing is prepared by applying coating layers composed mainly of a pigment and an adhesive on one side or both sides of a base paper. Such a coated paper for printing is classified into, for example, a cast-coat paper, an art paper, a coated paper, or a slight coated paper in accordance with the composition of coating layer, or the finishing process of a coated paper. Those coated papers applied with polychromatic or monochromatic printing processes have been widely used for commercial printed matter such as advertising leaflets, pamphlets, or posters, or for publications such as books or magazines. In recent years, a trend toward visualization and colorization of printed matter is in progress, which has increased demand for a high quality coated paper for printing with appropriate characteristics. Specifically, the level of printing gloss (ink gloss) after printing is emphasized.
When a small lot of commercial printed matter is prepared, an offset sheet-fed printing in which a sheet of coated paper is printed one by one is often used. However, it seems difficult for an offset printing to achieve a high ink gloss while improving the print efficiency by refining a coated paper for printing. When the ink settability (absorption of printing ink) increases, the drying rate of ink transferred onto a coated paper also increases, which improves printing efficiency. However, an increase of the ink settability of a coated paper for printing usually results in decreased printing gloss due to the following reasons:
In general, the main components of ink used for offset sheet-fed printing are a pigment, resin, solvent (normally, petroleum solvent), and vegetable oil (drying oil, semi-drying oil, etc.). When such an ink is transferred onto a coated paper, the viscosity of the ink increases as solvents in the ink are absorbed in gaps of a coating layer, so that the ink on the paper surface eventually becomes non-sticky (a status in which ink has been set) when a finger touches it. As time passes, the ink set on a paper surface is polymerized as vegetable oil composing the ink responds to oxygen in air so as to form a rigid dry coating film (a status in which ink has been dried). In this status, such an ink film will not be peeled off or damaged if external force is applied to some degree.
Namely, the rate of ink setting mainly relies on the ink settability of a coated paper (absorption ability of ink solvent), and the rate of ink drying mainly relies on composition of the printing ink itself.
Therefore, if the ink settability increases, the ink transferred onto a coated paper can dry sooner. However, in offset printing, the transfer of ink onto a coated paper surface is performed through a blanket cylinder. In particular, as an ink layer transferred onto a surface of a blanket cylinder is pulled out from the blanket cylinder and transferred onto coated paper, the ink layer just transferred onto the coated paper presents a split pattern. If a transferred ink has sufficient flowability, this split pattern will be dissolved to become a smooth pattern over time so as to achieve a good ink gloss. However, in a coated paper with high ink settability, as the flowability of the ink transferred onto such a coated paper can be lost quickly, the ink layer often dries up without dissolving the split pattern, so that it is difficult to obtain printed matter with a high ink gloss.